The invention relates to a printing unit for a printing machine, having an image cylinder, an image generating device for setting an image on the peripheral surface of the image cylinder, and an image transfer cylinder which transfers the image from the image cylinder to a printing substrate, the image transfer cylinder having a resilient cover which exhibits a deformation in the force transmission areas, and a transport belt that carries the printing substrates, supported by a back-pressure cylinder, driving the image transfer cylinder and the latter driving the image cylinder by friction.
A printing unit is shown in DE 199 34 658. The configuration of this printing unit compensated for the non-roundness of the driven image cylinder by an appropriate configuration and dimensioning of the image cylinder, image transfer cylinder and the resilient cover of the latter. In addition to compensating for non-roundness of the image cylinder, however, in such printing unit there is the problem that the transmission ratio of the drive also changes because, between the transport belt and image transfer cylinder, printing substrates and printing-substrate-free gaps alternate, as a result of which the radius which is definitive for the transmission ratio changes, and therefore a discontinuity in the image cylinder speed is likewise caused. The same can also occur as the result of non-roundness of the image transfer cylinder. The last-named fault cause may be counteracted, in exactly the same way as the non-roundness of image cylinders, by high precision in the roundness of the cylinders; however, there is no such possibility with respect to the change in the radius arising from printing substrates.
The invention is therefore based on the object of developing a printing unit in such a way that a continuous image cylinder speed is achieved. According to the invention, the object is achieved by a back-pressure cylinder being mounted such that it can be displaced with respect to the image transfer cylinder with a travel-dependent pressing force, which is dimensioned such that the speed of the image cylinder remains constant with respect to changes in the effective radius for driving the image transfer cylinder.
The invention is based on the following facts: in the case of the drive by friction, two effects occur which change the transmission ratio. Firstly, transmission ratio is changed by the change in effective radii when there is a printing substrate between the transport belt and image transfer cylinder, or the radius of the latter exhibits fluctuations. Secondly, the transmission ratio is determined by the extent of the deformation of the resilient cover in the force transmission areas. One of these areas is between the transport belt, with or without paper, and the image transfer cylinder; the other is between the image transfer cylinder and the image cylinder. A deformation of this kind leads to the resilient cover, in order to pass through the nip between the image transfer cylinder and image cylinder or transport belt, assuming a higher speed than that which corresponds to the rotation of the image transfer cylinder.
The speed of the image cylinder is therefore determined by the speed of the transport belt and the influence of the elastic deformations of the cover of the image transfer cylinder at the two force transmission points. This influence is referred to as overdrive. Were this overdrive identical at both force transmission points, then the two overdrives would cancel each other out. With respect to the drive of the image transfer cylinder there would be a reduction in speed as compared with the transmission of force by rigid surfaces and, with respect to the drive of the image cylinder, there would be acceleration. However, the overdrives are different. The overdrive between the image transfer cylinder and the image cylinder is determined by the mounting of this cylinder and the pressing pressure resulting from this. The overdrive between the transport belt and the image transfer cylinder is determined by the pressing force of the back-pressure cylinder, since this causes the deformation of the resilient cover in the area of the contact between transport belt and image transfer cylinder.
Were the transmission ratios, that is to say the effective radii and the overdrives, constant, then it would be sufficient to take this into account in generating the images. In actual fact, these overdrives fluctuate. In order to compensate for the fluctuation in the overdrive between image transfer cylinder and image cylinder because of the non-roundness of the latter, the above-cited DE patent application specifies a solution, the compensation of the fluctuations of the overdrive between transport belt with backpressure cylinder and image transfer cylinder is the subject of the invention. Without the measure according to the invention, an enlargement of the radius of the image transfer cylinder, which occurs when a printing substrate is located between the transport belt and image transfer cylinder, would lead to the transmission ratio being changed in the direction of slower rotation of the image transfer cylinder, since the enlargement of the radius acts in this direction. In the case of a rigid or normal spring mounting, the pressing force also increases in this case, and therefore so does the overdrive, which likewise leads to slower rotation of the image transfer cylinder, that is to say the error is increased further.
The invention is based on the finding that a configuration is needed in which the overdrive decreases with enlarged radius, it then being possible for this decrease in the overdrive to be adjusted in such a way that the speed reduction of the image transfer cylinder, and therefore of the image cylinder resulting from the enlarged radius, is opposed by a speed increase, compensating for this, as a result of a decreasing overdrive. This then balances out the effects of each radius enlargement, irrespective of whether this is attributable to the presence of a printing substrate in the transfer area or whether this is based on non-roundness of the image transfer cylinder. In this way, it is possible to achieve the situation where the image cylinder speed is independent of the influences just mentioned and therefore becomes constant. The influence of non-roundness of the image cylinder may be compensated for, to the extent that it is relevant, by the measures of DE 199 34 658, it being possible for these measures to be combined with the measures of this invention.
The advantage of the measure according to the invention is in the fact that the compensation adjusts itself, and neither a loss of quality nor compensation in the image generation has to be tolerated. The latter would be a complicated and expensive control system, particularly since the changes in the effective radius would have to be registered and taken into account without delay.
Exact dimensioning of the travel-dependent pressing force is achieved if the product of the transmission ratio which results from the respective effective radius and the transmission ratio which results from the respective deformations of the resilient cover in the force transmission area between the transport belt, with or without printing substrate, and the image cylinder remains constant. This dimensioning is the optimum, this being achieved more or less exactly, depending on the configuration of the invention. As a rule, a tolerance is predefined, within which the fluctuations are permissible, the tolerance depending on the respective quality requirements on a print.
Provision is expediently made for a force element to generate the travel-dependent pressing force, it being possible for a force element of this type to be configured in an extremely wide range of ways.
One proposal is that the force element is a controlled actuating element, data for this control system then having to be available. This data can be determined, for example, by inputting a substrate thickness. However, it is also possible for a sensor to be provided for registering the substrate thickness, and also a control system, the latter determining the pressing force with the aid of the substrate thickness. The actuating element can, for example, be designed in such a way that it acts on the piston/cylinder arrangements operated by a medium. These can be pneumatic or hydraulic cylinders.
A particularly simple and cost-effective configuration provides for the force element to be springs, which act on the backpressure cylinder with the pressing force. The advantage of this configuration is that the compensation can be achieved with simple mechanical devices, without any outlay on control. Use is preferably made of springs, which, at least in one characteristic-curve zone, have a falling force/travel characteristic. Although such a characteristic-curve zone will as a rule not achieve the optimum to one hundred percent, the prescribed tolerances can generally be achieved in this simple way. The springs expediently each have an adjusting element, with which the characteristic-curve zone can be set as a working zone. By such a setting, the optimum working zone within the force/travel characteristic curve can also be found, in order to set the pressing force as optimally as possible. With respect to the springs, it is proposed that these be disk springs, since such disk springs exhibit the aforementioned characteristic-curve zones. In this regard, reference is made to the xe2x80x9cDubbelxe2x80x9d Mechanical Engineering Pocket Book, 18th Edition G56, FIG. 7.
One further possibility for configuring the force element is that this is a lever-mounted weight. If, in the case of such a weight, the effective lever arm is shortened as a function of the travel, by an appropriate configuration of the lever in one travel direction, then it is likewise possible for a falling force/travel characteristic curve of this type to be achieved. This is only one example; various configurations of force elements with levers are possible, if appropriate with different transmission ratios.
Of course, the force element may also be a combination of at least two force-generating elements. For example, it is possible to combine a spring with a normal characteristic curve with a lever-mounted weight. In this case, the combination is expediently made in such a way that the sum of the force/travel characteristic curves of the force-generating elements result in at least one falling characteristic-curve zone, which is used as the working zone. Of course, a combination of passive and active force elements is also possible.